B737 Captain reports his First Officer descending below glide slope during an ILS approach in VMC. The First Officer is instructed to get back on the glide slope and lands safely beyond the displaced threshold.

Narrative

Visual approach to Runway 30 which has a displaced threshold of 1;443 FT. The First Officer was flying. He had done a RNAV approach with no problems and right on the glide path on the previous leg. He started this approach and flew the glide slope till about 400 FT. Then he started to go lower on the glide slope. I called 1/2 a dot low then 1 dot low. The PAPI lights had gone now to 3 red and one white I told him get back to the glide slope. He told me he was visual! I told him again to get back to the glide slope. He added power and got to between 1 dot to half a dot low. We cleared the runway displaced threshold at touch down. When we turned off I stopped the aircraft and I told him don't ever do that again! He now agreed. This type of approach activity is derived from the FAA and airline concept that the crew is responsible for clearance with terrain on a visual approach. Whenever you go below the glide slope or glide path in a transport category aircraft you open your self up for an accident. This should be changed in Part 121 that you never go below a glide slope; glide path; PAPI; or VASI lights on any visual approach. I have lost this argument in the past as the airline and the FAA will just shrug and brush it off.